This invention relates to a low amylopectin starch having unique molecular and functional characteristics. The starch may be obtained from a number of sources, including a novel corn breeding population comprising germplasm selections which are homozygous for the recessive amylose extender (ae) gene and into which ae modifier genes have been accumulated by recurrent selection.
High amylose starches have been known for many years. Such starches are derived from unique breeding sources, such as high amylose corn inbreds and their hybrids, wrinkled pea cultivars, high amylose barley cultivars, and the like. Starches obtained from these sources have been exploited commercially for their unique functional properties, such as superior film-forming ability, higher cooking temperature, higher gel strength, improved water resistance, and other properties attributed to the higher amylose content, relative to the amylopectin content, of these starches. Amylose is a linear polymer; amylopectin a larger, branched polymer of glucose.
The amylose content of starches obtained from nature varies widely, but remains less than one-third of the total starch content for the majority of starches derived from agricultural crops. The balance of the starch is generally described as amylopectin.
To date, commercially available high amylose starches have been marketed as either 50% amylose or 70% amylose starch. A range of about 40-75% amylose has been reported for commercial products. The existence of high amylose corn starches containing up to 88% amylose has been disclosed in scientific publications. However, it is generally accepted by persons skilled in the art that early methods used to isolate and analyze high amylose starches for amylose content were flawed, and early reports of amylose content in excess of 70% are viewed with skepticism. Amylose content has been described as "apparent amylose" because branched molecules having long outer chains yield an over-estimation of amylose content by potentiometric iodine analysis. See, Shannon, J. C. and Garwood, D. L., Chapter 3 in Starch: Chemistry and Technology, 2nd Ed., Whistler, R. L., BeMiller, J. N., and Paschall, E. F., Eds., Academic Press, Orlando, Fla. (1984) pp. 54-59. This publication also suggests that the presence of low molecular weight amylose may result in an underestimation of amylose content. In addition to experimental error, the location of the kernel on the ear, growing conditions, growing location and other plant variables are known to affect amylose content. Adverse growing conditions (e.g., drought) are known to reduce amylose content by as much as 7.7% in starch extracted from the grain of high amylose corn plants. See, e.g., V. L. Fergason, et al., Crop Sci, 5:169 (1965).
As more reliable means of analysis have been developed, the commercial high amylose starches derived from corn hybrids have been shown to contain at least 25% amylopectin, and, typically, no more than 75% amylose determined by potentiometric and colorimetric iodine analysis.
In the 1960's and early 1970's, C. T. Greenwood, G. K. Atkins, W. Banks and others, collaborated on a study of the chemical structure of the starches obtained from high amylose plant sources. The work included fractionation of the high amylose corn starch into various molecular components and physical, enzymatic and chemical analyses of the fine structure of the starch. No exclusion chromatography or other, similar, molecular weight analyses of amylopectin content were reported. The collaborators used commercially available starches having an "apparent amylose" content of about 50-75%. Their work is reported in the following publications:
Greenwood, C. T., and J. Thomson: Chem. Ind. (1960), 1110; PA1 Greenwood, C. T., and J. Thomson: Biochem. J. 82 (1962), 156; PA1 G. K. Adkins and C. T. Greenwood, Starke, 18 (1966) 171; PA1 Greenwood, C. T., and S. MacKenzie: Carbohydrate Res. 3 (1966), 7; PA1 Adkins, G. K., and C. T. Greenwood: Starke 18 (1966), 237; PA1 Adkins, G. K., and C. T. Greenwood: Starke 18 (1966), 240; PA1 Adkins, G. K., and C. T. Greenwood: Carbohydrate Res. 3 (1966), 81; PA1 Adkins, G. K., and C. T. Greenwood: Carbohydrate Res. 3 (1966), 152; PA1 Banks, W., and C. T. Greenwood: Carbohydrate Res. 6 (1968), 241; PA1 Adkins, G. K., and C. T. Greenwood: Carbohydrate Res. 11 (1969), 217; PA1 Adkins, G. K., C. T. Greenwood and D. J. Hourston: Cereal Chem. 47 (1970), 13; PA1 Banks, W., C. T. Greenwood and D. D. Muir: Starke 23 (1971), 199; PA1 Greenwood, C. T. and D. J. Hourston: Starke 23 (1971), 344; and PA1 Banks, W., C. T. Greenwood and D. D. Muir: Starke 26 (1974), 289 (a review of the work in this area).
In their publications, the collaborators identified the presence of a lower molecular weight starch constituent in addition to the normal amylose and amylopectin constituents. The collaborators reported that although the composition of this lower molecular weight fraction was not completely understood or characterized, the fraction appeared to be a low molecular weight, linear, amylose-like component. This fraction formed a complex with iodine that was typical of amylose (and atypical of amylopectin). The collaborators were uncertain as to whether the low molecular weight fraction contained branched molecules. Amylose ("normal amylose") content did not exceed 65%, even in a sample with an "apparent amylose" content of 75%. The collaborators disclosed difficulty in fully solubilizing the high amylose starches. They suggested that the amylose content of the starches reported in the literature had been overstated and noted that the error inherent in iodine affinity measurements of amylose content results in an inability to confirm iodine binding results using measurements by other techniques, such as butanol complexing and fractionation, or enzymatic characterization. However, they concluded from butanol fractionation, iodine binding and enzyme studies that an amylopectin content as low as 4% of the total starch had been observed in a "75% apparent amylose" (65% "normal amylose") starch.
Similar difficulties in measuring amylose content (both in ordinary and high amylose starches) have been reported elsewhere. See, e.g., A. H. Young, Chapter 8 in Starch: Chemistry and Technology, supra, pp. 249-265, 274-277 and 282.
It has now been discovered, by using more reliable measuring techniques, (e.g., full starch dispersion followed by butanol fractionation and exclusion chromatography) that a starch containing in excess of 75%, optionally in excess of 85%, amylose (normal amylose), together with about 8 to 25% low molecular weight amylose-like materials and less than 10%, optionally less than 5%, amylopectin may be extracted from a novel corn breeding population having a recessive amylose extender gene. The corn population is a genetic composite comprising germplasm selections which are homozygous for the ae gene and into which ae modifier genes have been accumulated by recurrent selection. The "ae modifier genes" are minor genes which interact epistatically with the ae gene to produce high amylose seed starch. It has been discovered that this "ae starch" has unique functional properties when compared to high amylose starch having 50 or 70% amylose, as well as when compared to ordinary corn starch, or waxy maize (100% amylopectin) starch.
It has further been discovered that if a starch blend is prepared from non-ae starch fractions so as to minic the molecular composition of the ae starch derived from the corn breeding population, then such a starch blend exhibits similar functional benefits in various applications.
Finally, it has been discovered that a starch having in excess of 75% amylose and less than 10% amylopectin has unique functional properties which can be exploited in many commercial applications, where starches and other polysaccharides and polyhydric alcohols traditionally have been used.